In recent years, there has been an increasing demand for quick-curing adhesives capable of being cured in a short period of time at room temperature, from the viewpoint of energy saving, labor saving, saving of resources, etc. As heretofore well known room temperature quick-curing adhesives, a quick-curing epoxy adhesive of two-part liquid system, an .alpha.-cyanoacrylate adhesive, an anaerobic adhesive and an acrylic adhesive of two-part liquid system may, for example, be mentioned.
Among these adhesives, the quick-curing epoxy adhesive of two-part liquid system is designed to be used by measuring and mixing two liquids i.e. a main agent and a curing agent. If the measurement or the mixing of the two liquids is not conducted adequately, it is likely that a substantial deterioration in the bonding strength will result. Further, even if the measurement and the mixing of the two liquids are adequately conducted, there still remain drawbacks such that the peel strength and impact strength are low and unsatisfactory.
The .alpha.-cyanoacrylate adhesive provides excellent operation efficiency, but is generally poor in the peel strength and impact strength and inferior in the heat resistance and moisture resistance. Therefore, its application field is very much limited. The anaerobic adhesive is an adhesive of the type which is cured by shutting off air by pressing it between objects to be bonded. Therefore, at the time of bonding, the adhesive is likely to be pressed out from the objects, and the adhesive at the portion which is in contact with air, such as the pressed out portion, will not be cured. Thus, the anaerobic adhesive will not be sufficiently cured, and deficient bonding is likely to result in a case of porous objects or in a case where the clearance between the objects is large.
The acrylic adhesive of two-part liquid system is generally called a second generation acrylic adhesive (SGA). It is of a two-part liquid system, but it has excellent operation efficiency in that it is unnecessary to accurately measure the two liquids, and it is capable of being cured quickly in from a few minutes to a few tens minutes by extremely rough measurement and mixing (in some cases, merely by contacting the two liquids). Yet, high peel strength and impact strength are obtainable, and the curing of the adhesive pressed out from the objects, is excellent. Therefore, adhesive of this type has been widely used. However, from the viewpoint of the energy saving, labor saving and saving of resources, there is an increasing demand for further shortening the curing time, which is presently from a few minutes to a few tens minutes at room temperature with such an acrylic adhesive of two-part liquid system.
Various proposals have been made to shorten the curing time of such an acrylic adhesive of two-part liquid system. For example, U.S. Pat. Nos. 4,348,503 and 4,429,088 disclose that an adhesive composed of a first liquid comprising an acrylic ester monomer, an aromatic perester, an organic acid and a transition metal, and a second liquid comprising an amine/aldehyde adduct activator, can be cured in a fixture time of 45 seconds.
Japanese Unexamined Patent Publication No. 199085/1985 discloses that also an adhesive composition comprising a certain specific urethane prepolymer, a (meth)acrylate monomer (referring generally to acrylates and methacrylates), a peroxy ester and an accelerator made of a compound which is capable of forming a redox system with the polymerization initiator of a peroxy ester, can be cured in a short fixture time within one minute.
Further, Japanese Unexamined Patent Publication No. 65277/1980 discloses that an adhesive composition having chlorosulfonated polyethylene and a certain specific acrylic monomer blended in certain specific proportions and comprising cumene hydroperoxide, an aldehyde-amine adduct activator and a transition metal introduced as any appropriate organic compound, can be cured in from 5 to 10 seconds.
On the other hand, it has been known to incorporate an acid phosphate compound to an acrylic adhesive in order to improve the adhesion to aluminum, stainless steel or chromated zinc coated steel parts. For example, Japanese Unexamined Patent Publication No. 132234/1976 discloses that an acid phosphate compound is incorporated to an anaerobic adhesive composition.
Further, U.S. Pat. Nos. 4,223,115 and 4,452,944 disclose that phosphorus compounds are incorporated to acrylic adhesives. However, U.S. Pat. No. 4,223,115 states that "the addition of phosphorus-containing compound to polymerizable acrylic adhesive compositions has a retarding effect which is directly proportional to the amount of such compound", and it is suggested that to solve such a problem, a tertiary amine having a certain specific structure is added to accelerate the curing.
Further, U.S. Pat. No. 4,731,146 discloses an improvement in adhesion for bonding metals such as aluminum and a curable primer solution in a volatile organic solvent of an acid phosphate compound and a certain specific metal salt. Also here, it is stated that phosphate compounds have been known as stabilizers for acrylic monomer compositions, and acid phosphate monomers have a cure inhibiting or slowing effect.
As described in the foregoing, quick-curing acrylic adhesives have been known, and acrylic adhesives having acid phosphate compounds incorporated to improve the adhesion for bonding metals, have been well known. However, since acid phosphate compounds have a cure slowing effect, there has been no adhesive having excellent adhesion to metals and simultaneously having a high curing speed.